This invention pertains to moisture sensors and, in particular, to moisture sensors utilizing optical techniques.
With the advent of optical fibers, assemblies which utilize such fibers in conjunction with optical techniques have been proposed for sensing the moisture condition of a location. U.S. Pat. No. 4,634,856 discloses a number of such fiber optic moisture sensing assemblies.
In one assembly disclosed in the '856 patent, a reflective target whose reflectance changes with moisture is situated at the end of an optical fiber and a light source, such as, for example, a laser generates light for coupling along the fiber to the target. By monitoring the light reflected from the target, an indication of the moisture content of the environment of the target can be ascertained.
In a second assembly described in the '856 patent, the optical fiber itself is adapted so that the light transmission properties of the fiber change with changes in moisture. Accordingly, by monitoring the change in light transmitted through the fiber, an indication of moisture conditions can be determined.
The '856 patent discloses various ways of adapting an optical fiber to make it moisture sensitive. One way is to select the fiber cladding from materials whose refractive index varies with moisture content. Another way is to induce imperfections in the fiber, such as splices or microscopic holes which will have different optical qualities depending upon their moisture content. Still another technique is to use two coupled fibers, with the amount of light coupled between fibers being a function of the moisture content of their cladding materials.
While the '856 patent describes general techniques for achieving fiber-optic moisture sensors, other fiber-optic moisture sensors have been proposed specifically for use with fiber optic communication or transmission lines. These sensors are disposed at closures along their respective transmission line and their fibers are adapted to undergo mechanical bending or distortion in response to moisture or water entry into the associated closure. In particular, the fibers are bent or distorted by elements which change dimension as a function of a change in water content. Moisture sensors of this type are disclosed, for example, in the following publications: "Optical Fiber Cable With Submersion Sensor Fiber", H. Sawano, et. al., International Wire and Cable Symposium Proceedings, 1987, pgs. 284-289; "A New Nonmetallic and Waterproof Optical Fiber Cable With Absorbent Polymer Ribbon, Kukita, et. al., International Wire and Cable Symposium Proceedings, 1987, pgs. 357-371.
The above moisture sensors designed for use with fiber-optic transmission lines have been developed to overcome certain disadvantages encountered in present day transmission systems. In particular, in systems in use today, so called "all filled" fiber cables are used for added moisture resistance. Additionally, at closure locations two closures are normally required. An inner closure houses the fiber cable, while an outer closure houses a water repellant encapsulation which surrounds the inner closure.
By using an appropriate moisture sensor at the closure locations, the aforesaid outer closure and encapsulation can be dispensed with. Furthermore, the fiber cable need no longer be an "all filled" cable. The result is that the size and cost of the closure and the cost of installation of the closure can be decreased. Also, the maintenance of the closure is made easier.
While the above advantages can thus be realized by using suitably designed optical moisture sensors at the closure locations of a fiber-optic transmission line, the sensors discussed above which rely on a changing the characteristics of the optical fiber itself with changes in moisture may not be entirely practical or easily realizeable. Accordingly, there still remains a need for an optical moisture sensor which can be readily realizeable from existing assemblies and which has a rapid response.
It is therefore a primary object of the present invention to provide an optical moisture sensor which can be readily realized and which is rapidly responding.
It is a further object of the present invention to provide an optical moisture sensor which is readily adaptable from existing assemblies.
It is yet a further object of the present invention to provide an optical moisture sensor which can be readily adapted for use in closures of a fiber optic transmission line.